Conventionally, in a communications network such as a cell phone network, a position of a mobile terminal is determined by a delay time of a signal from a base station. For example, the mobile terminal calculates the delay time of the radio signal from the base station and transmits resultant information to a position server. The position server calculates the position of the mobile terminal. The mobile terminal then obtains its position information from the calculated result via the communications network and determines a position thereof.
To determine the position of the mobile terminal, it is necessary to memorize almanac information of the base station and execute complex calculations based on the almanac information. Therefore, normally, the mobile terminal does not execute the complex calculation. That is, the mobile terminal obtains information for the complex calculation and makes the position server execute the complex calculation based on the information therefrom.
The position of the mobile terminal is calculated as follows. FIG. 8 is a schematic view showing communications with a CDMA (Code Division Multiple Access) transmission system. Respective base stations A-E are determined by PN (Pseudo Noise) offset assigned to each base station. The mobile terminal MS selects not only a communication target base station A communicating therewith but also a reference base station. For example, because the reference base station is used as a time reference for calculating the position of the mobile terminal MS, base station C is selected, as it has the shortest radio signal delay time among all base stations.
The mobile terminal MS transmits position information, which includes the PN offsets indicating position information of the base stations A-E (e.g., PN 65 is the PN offset of the base station A, PN 384 is that of the base station B and PN 256 is that of the base station C in FIG. 8) and calculated delay times of respective base stations A-E relative to the position server.
The position server extracts the latitudes and longitudes of respective base stations A-E based on the position information (i.e., respective PN offsets) from the mobile terminal MS with base station almanac information prepared beforehand. The position server calculates distances between each base station and the mobile terminal MS based on relative values thereof defined by the signal delay time to the time reference when the signal delay time of the base station C is the time reference. The resultant distances are retransmitted to the mobile terminal MS.
A region shown within a circle in the figure (hereinafter referred as serving region) is defined as a predetermined radius from a serving base station for determining the reference base station by the position server. If the position server confirms the base station that is selected as the reference base station by the mobile terminal MS within the serving region, the position server determines that the base station is the reference base station and the time reference for a position calculation. The radius of the serving region is defined with respect to each base station and memorized in the base station almanac information stored in the position server. Incidentally, the arrows shown in the figure indicate the relationships by which the mobile terminal MS obtains base station information from the other base stations.
Since the PN offsets are defined by finite numbers (i.e., 128 to 512 species), plural base stations having identical PN offsets are sometimes assigned in the same serving region. In this case, it is impossible to determine the correct reference base station from the plural base stations with the same PN offset. Therefore, the position calculation is erroneous because the signal delay times of the plural base stations are different from each other.
When the mobile terminal MS is moved from the position shown in FIG. 8 to the position shown in FIG. 9, the mobile terminals MS select the nearest base station therefrom as the reference base station. Accordingly, the base station E having PN offset 128 is selected as the reference base station. However, base station D has a PN offset 128 in the serving region. Therefore, the position server must select not the base station A selected by the mobile terminal MS but the base station D. In this case, as mentioned above, a position calculation error is caused because the signal delay times of the plural base stations are different from each other.